1. Field of the Invention
The present invention relates to a process for preparing a semiconductor substrate comprising a substrate having a metallic surface or a metallic substrate, and a monocrystalline semiconductor thin film layer having outstanding crystallinity, laid thereon.
2. Related Background Art
Formation of a monocrystalline Si semiconductor layer on an insulator is widely known as a silicon-on-insulator (SOI) technique and has been extensively studied because devices based on utilization of the SOI technique have many advantages that have not been obtained in case of bulk Si substrates for preparing ordinary Si integrated circuits. That is, the following advantages can be obtained by utilizing the SOI technique:
1. Easy dielectric isolation with a possibility of higher level integration PA0 2. Distinguished resistance to radiation PA0 3. Reduced floating capacity with a possibility of higher speed PA0 4. Omission of well formation step PA0 5. Prevention of latch-up PA0 6. Possibility to form a fully depleted field effect transistor by thin film formation, etc. PA0 1. After the surface oxidation of a Si monocrystalline substrate, windows are made to partially expose the Si substrate, and a Si monocrystalline layer is formed on the SiO.sub.2 by epitaxial growth of Si in the lateral direction, while utilizing the exposed Si substrate as seed. In this case, deposition of the Si layer on the SiO.sub.2 is made. PA0 2. A Si monocrystalline substrate itself is utilized as an active layer and SiO.sub.2 is formed as its underlayer. In this case, no Si layer is deposited.
To obtain the above-mentioned many advantages of device characteristics, processes for forming the SOI structure have been studied for for at least 20 years. The results are summarized, for example, in the following literature: Special Issue: "Single-crystal silicon on non-single crystal insulators", edited by G. W. Cullen, Journal of Crystal Growth, Volume 63, No. 3, pp. 429-590 (1983).
In the past, SOS (silicon-on-sapphire) formed by heteroepitaxial growth of Si on a monocrystalline sapphire substrate by chemical vapor deposition (CVD) of Si was disclosed as a successful result of most matured SOI techniques, but its wide application was interrupted by the occurrence of many crystal defects due to lattice mismatching at the interface between the Si layer and the underlayer sapphire substrate, by diffusion of aluminum into the Si layer from the sapphire substrate, and largely by a high cost of the substrate and delay in formation of larger area.
Recently, some attempts have been made to form an SOI structure without using the sapphire substrate. The attempts can be classified into the following two major groups:
Recently, solar cells have been utilized in many fields, and solar cells of high efficiency and low cost have been in demand. Particularly since solar cells on a metallic substrate are available at a relatively low cost and can have readily a larger area, a process for forming a semiconductor layer of good crystallinity on a metallic substrate at a low cost has been desired for increasing their efficiency.
A light-transmissible substrate is important for making a contact sensor as a photo receptor element or a projection-type liquid crystal image display apparatus. A driving element of very high performance is required for making the sensor or image elements (picture elements) of the display apparatus to have a higher density, a higher resolution and a higher detail. As a result, it is necessary to use a monocrystalline layer having an outstanding crystallinity as an element to be provided on the light-transmissible substrate.
However, the SOI element utilizes an insulating substrate of low heat radiation as its underlayer, and thus has a poor heat radiation, as compared with the bulk element formed on a bulk substrate and has such problems as interruption of development of practical finer and higher speed circuits.
With finer elements, the contact size has been made finer, resulting in increasing contact resistance, or when the individual wirings are to be shielded to prevent cross-talk between the wirings, the area occupied by the wirings is increased, resulting in failure to satisfy the requirements for making the element finer.
When making a bipolar transistor which is to work at a higher speed, the current process for impurity diffusion has a limit in making a collector embedded layer which has a lower resistance, and particularly making an increase in the resistance of collectors in series connection is a serious problem.
Recently, solar cells have been utilized in many fields, and solar cells of high efficiency and low cost have been in demand. Particularly since solar cells on a metallic substrate are available at a relatively low cost and can have readily a larger area, a process for forming a semiconductor layer of good crystallinity on a metallic substrate at a low cost has been desired for increasing their efficiency.
However, in case of amorphous silicon, it is hard to increase the efficiency, and in case of Ga--As monocrystal, it is hard to reduce the cost. That is, the preparation of a solar cell of high efficiency and low cost on a metallic substrate has not been fully realized.
Generally, only an amorphous layer or at best a polycrystalline layer is formed on a light-transmissible substrate due to the randomness of its crystal structure, and a device of high performance cannot be prepared. That is, since the crystal structure of the substrate is amorphous, a monocrystalline layer of good quality cannot be obtained merely by depositing an Si layer thereon.
Furthermore, amorphous Si or polycrystalline Si has many defects in the crystal structure, and it is difficult to prepare a driving element capable of satisfying current or future requirements.